This is a critical step that requires some care and, possibly, some
experimentation. The aim is to remove the large number of telluric
lines that appear in IR spectra. This is done by dividing the
object spectrum with that of a telluric standard. Since this is a
division of one spectrum by another, it is important that the
strength, shape and centroid of the telluric lines match.

First and foremost, the telluric standard and the object have to
observed with the same instrument setup, with roughly the same airmass
and, if possible, consecutively.

Secondly, the object and science data should be reduced in the same way and
with the same calibration frames.

For the best results, one may have to modify the spectrum of the telluric
standard so that the center and strength of the telluric lines match those
of the object spectrum.

The next step is to remove spectral features that
have been imprinted onto the object spectrum from the telluric standard
itself.

Telluric standards are either hot stars or solar type dwarfs.
Both types contain spectral features that should be removed. For
solar type stars, one can use the observed solar spectrum to remove the
features. This can be tricky if the spectral resolution of the instrument
is a function of the wavelength (which is the case for ISAAC) as it means
that the kernel for convolution also has to be a function of wavelength. The
arc spectra and the OH lines can be used to estimate what this function is.

Hot stars usually contain helium and hydrogen lines. If the spectral
regions around these lines are of interest, then one should think
carefully about using these type of stars. If the resolution is high
enough, which is certainly the case for MR observations, one can try
to remove these lines by fitting them. Alternatively, one can use a
second telluric standard that does not have helium or hydrogen lines
so that these lines can be removed from the hot star.

The ISAAC FITS header does not always contain the full target name of
the telluric standard and, sometimes, operators forget to include it
in the night report. To find out which telluric standard was used,
look at the RA and DEC of the target and consult either the
ISAAC manual
or the list of
Hipparcos
stars that are often used as telluric standards.